Mumbai |
OPTICAL FIBER MANUFACTURE
INTRODUCTION :
Optical fibers play a vital role in modern communication. If we consider the history of optical fiber, image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s.
In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber. Modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade. Development then focused on fiber bundles for image transmission.
The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956.
In 1965, Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables (STC) were the first to promote the idea that the attenuation in optical fibers could be reduced below 20 decibels per kilometer, allowing fibers to be a practical medium for communication. They proposed that the attenuation in fibers available at the time was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering.
The crucial attenuation level of 20 dB/km was first achieved in 1970, by researchers Robert D. Maurer, Donald Keck, Peter C. Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now Corning Incorporated. They demonstrated a fiber with 17 dB/km attenuation by doping silica glass with titanium. A few years later they produced a fiber with only 4 dB/km attenuation using germanium dioxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet.
Attenuations in modern optical cables are far less than those in electrical copper cables, leading to long-haul fiber connections with repeater distances of 50–80 km (30–50 miles).
BASIC PRINCIPLE :
When light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical angle" for the boundary), the light will be completely reflected. This is called Total Internal Reflection.
MANUFACTURING :
There are a wide range of methods available for the manufacturing of optical fibers. All the methods of manufacturing optical fiber has been discussed below.
ROD IN TUBE METHOD:
This is one of the oldest methods of manufacturing optical fiber. It consists of a high quality fused silica rod and fluoride doped silica tube. The rod is placed inside the tube and sent in to a drawing furnace and heated. At the output we get the fiber of required diameter with core and cladding. Then the fiber is rolled on to rotating drum.
DOUBLE CRUCIBLE METHOD :
In this method, two platinum crucibles are placed inside a furnace. Fused silica is laced in the inner crucible and fluoride doped silica is placed in the outer crucible.
Now the furnaces are heated and hence the required fiber with cladding is obtained through the nozzle. This fiber is then rolled on to a rotating drum.
In the above two methods losses are very high. Hence they are not used to manufacture optical fibers.
VAPOUR DEPOSITION METHODS:
Vapour deposition method is most widely used to manufacture optical fiber. The above tree represents the various methods and processes involved in manufacturing optical fiber using vapour deposition method. In these methods perform is developed and then fiber is drawn from them. Let us see the different vapour deposition methods of developing preforms.
INSIDE VAPOUR DEPOSITION :
This process involves four steps. They are,
Forming of glass particles (soot) from the raw materials.
Sinterung of soot to a transparent glass.
Collapsing the tube to forn preform.
Fiber drawing from preform.
OUTSIDE VAPOUR DEPOSITION :
This method was introduced in 1973. It consists of oxygen – hydrogen burner. The burner consists of three orifices. Vapours of sicl4, o2, and dopants are sent through the central orifice. Fuel gas and o2 mixture are sent through the outer orifice. The other orifice is used to produce deposition on the mandrel. Mandrel is nothing but a rod made of Al2o3. The burner moves back and forth and the mandrel is slowly rotated during deposition. Single layer of silica is deposited per pass. Up to 1000 layers can be formed in this method. The deposition is about 80cm in length and 11cm in diameter and weighs 1800g. The deposition rate is 1 to 48 layers per minute.
After cooling the mandrel is removed from the glass soot. Then the porus deposition is vertically passed in to an annular furnace and heated to 1500degree Celsius to produce a dense glass rod. By using He atmosphere with few percentage of chlorine in the furnace, OH ions that produce distortions in the fiber can be eliminated.
VAPOUR PHASE AXIAL DEPOSITION:
In 1977, researches at Nippon telegraph and telephones in Japan introduced this technique. The two main advantages of this method are,
No collapsing is needed.
Highest bandwidth Graded Index Fiber can be obtained.
In this method performs up to 580kms can be drawn. The only difference between OVD and this method is, the deposition is made axially. It has three parts.
A water-cooled stainless steel box as the deposition chamber with the burner and exhaust at the lower part and sintering at the upper part.
The chemical delivery and exhaust systems are same as other methods.
Control system with controllers for position and rotation of perform and for temperature profiling.
The raw materials are vapourised and fed to the burner. The tiny glass particles from the burner are blown to the rotating silica rod. Thus the porus perform is formed during deposition. The distance between burner and perform must be constant to give uniform deposition. This is done by moving the perform upwards. For monitoring this, a video camera is used. The temperature can be measured using thermo viewer. The thermo viewer consists of a scanning pyrometer. The disadvantage of method is, this is not suitable for multiple clad single mode fiber.
MODIFIED CHEMICAL VAPOUR DEPOSITION :
This was developed by bell laboratories in 1974. The raw marerials used in this method are sicl4, Gecl4, Pocl3, Bcl3 or BBr3. Compared to normal CVD, concentration of reagents and temperature are high in this method.
Controlled amount of SiCl4 vapours and dopant along with O2 are fed in to a rotating fused silica substrate. It is heated using a oxy – hydrogen burner to 1600 degree celcius. Thus silica particles are deposited on the inner wall of the substrate. The burner is moved from left to right. When it gets to right end, it moves fast back to the left end. Since deposition takes place only when the burner moves towards the direction of the flow of gases. Refractive index can be by varying gas mixture. Using this method 40 to 100 iayers are formed. Then the tube is collapsed to give the preform.
PLASMA ENHANCED CVD :
It is an extension of MCVD. It combines the advantage of MCVD with large increase in deposition rate. An isothermal plasma (f==3MHZ) at atmospheric pressure is used. This was used first in 1977.
Plasma initiates the chemical reaction. Plasma is instable and hence performance is limited. Nowadays, plasma is used only to initiate chemical reaction and deposit the glass particles. A hydrogen burner is used for sintering process. RF generator and RF coils are additionally needed.
Diameter of the substrate tubes are increased as 46mm inside and 50mm for outside. With RF power of about 12KW, an oxygen RF plasma is built up at normal pressure. The plasma is centered at the glass tube. Its temperature is 10,000 degree Celsius. At the region of plasma, hot spots can be produced on the glass tube, which produces distortion. So, water is used continuously for cooling.
PALSMA ACTIVATED CVD:
Use of plasma for fiber optics was first established in 1975 by a research group of Phillips in Germany. PCVD is also a modification of MCVD. The substrate tube is placed in a stationary furnace at 1200 degree Celsius. Normally, rotation of the tube is not necessary. But practically, it is done in order to prevent distortions due to small temperature changes over the tube. Instead of a burner, a microwave cavity(f=2.5GHZ) traverses the furnace repeatedly. At a pressure of 10 to 50 Hpa, a non-isothermal or cold plasma is generated inside the tube. This initiates the deposition of transparent glass layer on the inner wall. Sintering is redundant. Deposition takes place on both directions. Up to 1000 layers can be deposited. It is more efficient than any other. The deposition is 80% with GeO2 and 100% with SiO2.
PLASMA OUTSIDE DEPOSITION :
This method has been introduced at Germany in 1975. A plasma torch is used for chemical reaction and deposition. Big transparent performs with more than 1kg can be obtained. No sintering or collapsing is needed in this method. The raw materials SiCl4, Cci2F2 and oxygen are fed in to plasma torch at 1800 to 2300 degree Celsius. The fluoride deposited silica are deposited as a transparent glass on the surface of a rotating glass rod (suprasil-W rod). Preforms with 60mm diameter and 200mm length can be fabricated. These performs are made in to rods of &m length and 10mm diameter. Then they are drawn in to fiber. The major disadvantage is OH ions are present in large amount and hence attenuation is high.
DRAWING PROCESS:
Preforms converted to fibers of diameter 100 to 400mm and 125mm is used as standard overall fiber diameter.A polymer coating is done here in order to protect the fiber from humidity and external mechanical forces. During drawing, the refractive index of the preform doesn’t change. The average height if the drawing towers used is 18m.
The maximum temperature of the furnace is 2200 degree Celsius. Temperature is maintained precisely because, a small change in temperature may vary the fiber diameter.
There are two types of furnaces, which are widely used. They are,
Graphite resistance furnace
Zirconium induction furnace
Graphite furnace can be easily controlled. A protective noble gas like argon is needed to prevent any degradation of burning if graphite heater. This noble gas can form fiber distortion.
Zirconium does not need protective gas. But, handling and temperature control is difficult.
In the apparatus for drawing, a diameter monitor is used below the furnace. Varying pulling velocity of the fiber varies diameter. Fiber is coated with a suitable polymer at the intermediate region. If pulling velocity of fiber exceeds 5m/s, the coating bath must be pressurized to minimize shear rates. This avoids slipping of fiber. Heating or applying UV rays then cures the polymer.
If the pulling speed is high, then the height of the tower must also be high. Varying the position of the coating nozzle can vary the coating concentration. Then it is wound through capston to drum.
COMPARISON:
From the above discussions, we may conclude that no method is a perfect one for manufacturing the optical fiber. Depending on the requirements, the type of manufacturing is chosen. More researches are going on regarding manufacture of perfect loss less optical fiber. Nowadays the use of optical fiber have been increased and it may be expected to contribute in most of the communication systems in future.
Website Design
0 comments:
Post a Comment